The present invention relates generally to electronic filters and more particularly to filters that include tunable varactors.
Electronic filters are widely used in radio frequency (RF) and microwave circuits. Tunable filters may significantly improve the performance of the circuits, and simplify the circuits. There are two well-known kinds of analog tunable filters used in RF applications, one is electrically tuned, usually by diode varactor, and the other is mechanically tuned. Mechanically tunable filters have the disadvantages of large size, low speed, and heavy weight. Diode-tuned filters that include conventional semiconductor varactor diodes suffer from low power handling capacity, that is limited by intermodulation of the varactor, which causes signals to be generated at frequencies other than those desired. This intermodulation is caused by the highly non-linear response of conventional semiconductor varactors to voltage control.
Tunable filters for use in radio frequency circuits are well known. Examples of such filters can be found in U.S. Pat. Nos. 5,917,387, 5,908,811, 5,877,123, 5,869,429, 5,752,179, 5,496,795 and 5,376,907.
Varactors can be used as tunable capacitors in tunable filters. Common varactors used today are Silicon and GaAs based diodes. The performance of these varactors is defined by the capacitance ratio, Cmax/Cmin, frequency range and figure of merit, or Q factor (1/tan δ) at the specified frequency range. The Q factors for these semiconductor varactors for frequencies up to 2 GHz are usually very good. However, at frequencies above 2 GHz, the Q factors of these varactors degrade rapidly. At 10 GHz the Q factors for these varactors are usually only about 30.
Varactors that utilize a thin film ferroelectric ceramic as a voltage tunable element in combination with a superconducting element have been described. For example, U.S. Pat. No. 5,640,042 discloses a thin film ferroelectric varactor having a carrier substrate layer, a high temperature superconducting layer deposited on the substrate, a thin film dielectric deposited on the metallic layer, and a plurality of metallic conductive means disposed on the thin film dielectric, which are placed in electrical contact with RF transmission lines in tuning devices. Another tunable capacitor using a ferroelectric element in combination with a superconducting element is disclosed in U.S. Pat. No. 5,721,194.
Commonly owned U.S. patent application Ser. No. 09/419,126, filed Oct. 15, 1999, and titled “Voltage Tunable Varactors And Tunable Devices Including Such Varactors”, discloses voltage tunable varactors that operate at room temperature and various devices that include such varactors. Commonly owned U.S. patent application Ser. No. 09/434,433, filed Nov. 4, 1999, and titled “Ferroelectric Varactor With Built-In DC Blocks” discloses voltage tunable varactors that include built-in DC blocking capacitors. These varactors operate at room temperatures to provide a tunable capacitance.
There is a need for tunable filters that can operate at radio frequencies with reduced intermodulation products and at temperatures above those necessary for superconduction.